This invention relates to a magnetic flowmeter amplifier system, and in particular to a multiple stage amplifier system having an improved automatic zeroing function.
In an electromagnetic flowmeter a magnetic field across a flow tube generates a voltage in a fluid flowing through the tube. The voltage, which is proportional to flow rate, is sensed by a pair of electrodes in contact with the fluid and amplified by a signal processing system to produce an output signal which is proportional to flow rate.
It is now common for the magnetic field to be generated by a pulsed direct current applied to coils associated with the meter. The meter therefore produces a train of output pulses separated by periods of zero output.
To maintain the output offset of the amplifier at a suitably low level, it is desirable to provide the amplifier with an automatic zeroing circuit, to drive the output to a zero reference level when its input level is nominally zero; that is, during the intervals between pulses when no signal representative of flow is present.
An automatic zeroing circuit typically includes a switch for switching the zeroing circuit into a feed-back loop around the amplifier and an integrating operational amplifier for driving the output of the amplifier to zero when the switch is closed. When the switch is open, the output of the integrating operational amplifier remains at the same voltage as long as the input remains the same, thereby maintaining the reference voltage of the amplifier at the same level.
The amplification of a magnetic flowmeter signal processing system is preferably carried out by a cascaded multiple stage amplifier system including at least a first amplifier and a second amplifier. When two amplifiers, each provided with such an automatic circuit, are cascaded, however, a substantial error (typically on the order of fifty millivolts) is introduced. An error of this magnitude is unacceptable in a magnetic flowmeter.